1. Field of the Invention
The invention relates generally to subsea exploration systems. More specifically, the invention relates to a marine electromagnetic measurement system and a method of deploying and retrieving the same. One application for electromagnetic measurement systems is as a marine magnetotelluric receiver.
2. Background Art
Magnetotelluric (MT) measurements are used to compute an electromagnetic impedance of selected earth formations. MT measurements are especially useful in regions where seismic imaging is inappropriate. For example, MT exploration is useful when evaluating geologic formations such as salts and carbonates. Salts, carbonates, and other particular formations may scatter seismic energy when seismic energy is propagated through them because of large velocity contrasts and inhomogeneties located within these formations, whereas the electromagnetic energy of the MT source fields propogates through these layers with less distortion. The MT method measures variations in the earth's magnetic and electric fields and does not use seismic energy to determine formation characteristics.
The MT method is typically used to measure an electromagnetic impedance as a function of frequency. Lower frequency provides a greater depth of penetration. The measured impedance may be transformed into an apparent resistivity and/or conductivity of the selected formations. Measuring impedance at several locations and at various frequencies enables a determination of resistivity and/or conductivity as a function of both depth and horizontal position. Therefore, the MT method may be used to evaluate formation resistivity over large areas of the seafloor. The formation resistivities of the various formations in a selected area may then be analyzed to determine the formation geometry, the presence or absence of hydrocarbons in selected formations, and the like.
The MT method is a passive method that uses natural variations in the earth's magnetic field as an energy source. The MT method includes a subsea system that detects orthogonal magnetic and electric fields proximate the seafloor to define a surface impedance. The surface impedance, as described above, may be measured over a broad range of frequencies and over a large area where layered formations act in a manner analogous to segments of an electrical transmission line. An MT method that operates according to the principles described above is generally disclosed in U.S. Pat. No. 5,770,945 issued to Constable. This type of electromagnetic receiver can also be used to record electromagnetic signals which originated from various kinds of transmitter systems such as a towed cable bipole or magnetic loop source.
In addition the receivers could be used to detect electromagnetic radiation originating from other types of signals such as emanating from naval ships (corrosion currents, electric circuits, generators, moving machinery) or from electric or magnetic sources located in boreholes or nearby land sources. The objective of these measurements could range from detailed exploration of the subsurface conductivity structure to monitoring naval traffic or operations to determining leakage signals from subsea cables.
The subsea system usually includes an apparatus such as an magnetotelluric (MT) measurement system 100 disclosed in the Constable patent and shown in FIG. 1. The MT measurement system 100 includes a body 102 having a battery pack (not shown), a data acquisition system 104, two orthogonally oriented magnetic sensors 122 and 124, and four arms 139, 140, 142, and 144, each of which includes an electrode 118, 119, 120, 121 mounted at the end thereof. The electrodes 118, 119, 120, 121 are silver-silver chloride electrodes, and the magnetic sensors 122, 124 are magnetic induction coil sensors.
The arms 139, 140, 142, 144 are five meters long and approximately 2 inches in diameter. The arms 139, 140, 142, 144 are typically formed from a semi-rigid plastic material (e.g., polyvinyl chloride or polypropylene) and are fixed to the body. The five meter length of the arms 139, 140, 142, 144 makes it difficult to store, deploy, and retrieve the MT system 100 from a surface vessel (not shown) because the arms 139, 140, 142, 144 are fixed with respect to the body 102 (as shown in FIG. 1). The arms 139, 140, 142, 144 are designed to rest on the seafloor when the MT system 100 is in place.
The body 102 is attached to a releasable concrete anchor 128 that helps the MT system 100 sink to the seafloor after deployment. The body 102 generally rests on top of the anchor 128 when it is positioned on the seafloor. The anchor 128 may be released after MT measurements have been completed so that the body 102 may rise to the surface and be retrieved by the surface vessel (not shown).
The system shown in FIG. 1, therefore, consists of two orthogonal electric dipoles and two orthogonal magnetic sensors. The magnetic sensors are located proximate the power supply and the data acquisition system. Because the magnetic sensors are very sensitive so as to detect small changes in the earth's magnetic field, the magnetic sensors may also detect equivalent magnetic fields generated by current flowing from the power supply to the data acquisition system and other electrical equipment. These equivalent magnetic fields may therefore contaminate the data and must be removed from the data using digital signal processing techniques.
Moreover, the magnetic sensors are extremely sensitive to noise. Any motion of the body and/or arms of the MT system caused by sea currents or marine life moving past the MT system as well as the motion of conductive fluid around the corresponding sensor can be detected. These fluctuations in the magnetic field are also recorded by the magnetic sensors and must be removed using signal processing techniques.
There is a need, therefore, for an MT system that is less sensitive to motion generated by subsea events and to equivalent magnetic fields produced by the flow of electric current from the power supply to, for example, the data acquisition system. Moreover, it would be advantageous to design a subsea MT system that is easy to store, deploy, and retrieve.